Light emitting diode (LED) arrays are becoming more popular as an image source in both direct view and virtual image displays. One reason for this is the fact that LEDs are capable of generating relatively high amounts of light (high luminance), which means that displays incorporating LED arrays can be used in a greater variety of ambient conditions. For example, reflective LCDs can only be used in high ambient light conditions because they derive their light from the ambient light, i.e. the ambient light is reflected by the LCDs. Some transflective LCDs are designed to operate in a transmissive mode and incorporate a backlighting arrangement for use when ambient light is insufficient. In addition, transflective displays have a certain visual aspect and some users prefer a bright emissive display. However, these types of displays are generally too large for practical use in very small devices, such as portable electronic devices.
Organic electroluminescent device (OED) arrays are emerging as a potentially viable design choice for use in small products, especially small portable electronic devices, such as pagers, cellular and portable telephones, two-way radios, data banks, etc. OED arrays are capable of generating sufficient light for use in displays under a variety of ambient light conditions (from little or no ambient light to bright ambient light). Further, OEDs can be fabricated relatively cheaply and in a variety of sizes from very small (less than a tenth millimeter in diameter) to relatively large (greater than an inch) so that OED arrays can be fabricated in a variety of sizes. Also, OEDs have the added advantage that their emissive operation provides a very wide viewing angle.
A problem in the use of OEDs in displays is the generation of the colors necessary to achieve a full color display. Red, green and blue OEDs can be fabricated but they require different organic materials and, thus, each color must be fabricated separately. Furthermore, the colors achieved are not a pure primary color, but have a relatively broad spectrum. Emission of red light is very difficult to achieve in OEDs however, it is known to convert other colors, such as blue light, to red light. One such technique is disclosed in Japanese Publication, Kokai Patent No. Hei 8-286033 entitled "Red Emitting Device Using Red Fluorescent Converting Film", published Nov. 1, 1996. While converting blue light to red light, the efficiency of the conversion is unacceptably low, and the red light contains unacceptable levels of blue green components.
Accordingly, it is highly desirable to provide apparatus and a method of converting broad spectrum light to red light.
It is a purpose of the present invention to provide a new and improved apparatus and a method of generating red light.
It is a further purpose of the present invention to provide apparatus and a method of efficiently generating red light.